Low-porosity carbon templates mitigate mass transport limitations in Fe-N-C catalysts

Fe-N-C catalysts, as promising non-precious metal alternatives for the oxygen reduction reaction (ORR), still suffer from severe mass transport limitations in proton exchange membrane fuel cells (PEMFCs) due to water flooding of active sites embedded in micropores. Although pore engineering through a selected template is a general strategy, the structural features of an ideal template, particularly those governing the exposure of active sites and thus affecting mass transport, remain elusive. Here, we demonstrate that low-porosity carbon templates maximize the ratio of active sites distributed at or near the surface, thereby enhancing their exposure and accessibility while reducing mass transport resistance during the ORR process. The C_lp-1@PPy and C_lp-2@PPy (PPy = polypyrrole) catalysts, derived from low-porosity carbon templates, achieve peak power densities of 0.96 and 1.03 W·cm⁻² under H₂/O₂ and 0.50 and 0.52 W·cm⁻² under H₂/air, demonstrating excellent performance in PEMFC tests. Structural and electrochemical characterizations reveal that the enhanced surface exposure of active sites effectively mitigates mass transport resistance during the ORR, thereby offering a general design principle for overcoming mass transport limitations in Fe-N-C catalysts for PEMFC applications.